This invention relates to determining the API gravity of an oil encountered within a subterranean reservoir. More particularly, the invention provides a relatively rapid method for determining the gravity of a relatively small amount of oil contained in or adsorbed on cores or bit cuttings or the like samples of a reservoir rock.
Determinations of oil gravities are known to be extremely valuable in the planning required during the development of an oil production prospect. The viscosity and volatility of a crude oil may vary quite widely with variations in API gravity, particularly with respect to gravities within about the 10.degree.-20.degree. API range. For example, in a typical heavy oil prospect, the change in viscosity which accompanies a change of from 11 to 12 degrees in API gravity is approximately 62 centipoises. But, the change in viscosity as the gravity changes from 18 to 19 degrees API is approximately only 3 centipoises. It is important to determine the oil gravity as precisely as possible and as early as possible in the predevelopment economic studies of an oil production prospect.
Previously used methods of determining the API gravity of an oil have included (a) direct measurements on oil samples recovered from production tests or samples obtained by formation fluid-sampling logging devices, or the like, (b) mud-gas chromatographic analyses, and (c) measurements of the refractive index (RI) of oil retorted from cores or samples of the reservoir formation. In the latter method, a calibration curve is prepared by retorting oils which have known gravities and similar chemical compositions and measuring the refractive indexes of the liquids condensed from the retorting. The gravity of the oil being tested is then determined by measuring the refractive index of its distillate and assuming that its gravity equals that of an oil of known gravity from which a distillate of similar refractive index is obtained.
The previously used methods have numerous disadvantages. Although the direct measurements are, of course, the most accurate, they are also the most expensive and time-consuming or require a relatively large sample. In a mud-gas analysis, due to variables which are apt to be encountered during drilling, the effects of various mud-gas components tend to be averaged out so that the measurements fail to provide usable API gravity data. Numerous ones of the lighter or more volatile components of a crude oil may be cracked or depolymerized during a retorting operation in which the vapors are condensed to a liquid. Because of this a property such as the refractive index of the distillate may vary from chemical composition and reactivity factors that are not related to the API gravity of the oil.